REDWOOD CITY — San Mateo County health officials announced Friday that norovirus was behind an outbreak of acute gastrointestinal illness at the upscale Hotel Sofitel in Redwood Shores.

Lab analysis of samples taken from some of the people who were sickened confirmed the highly contagious virus was to blame. At least 60 hotel guests and employees reported falling ill with symptoms that included vomiting and diarrhea, beginning Oct. 26, county health spokeswoman Robyn Thaw said.

It’s not clear yet how the virus was spread, but norovirus is the leading cause of disease outbreaks from contaminated food in the United States, according to the Centers for Disease Control and Prevention. Thaw said the hotel told county health investigators that guests at three separate special events became sick.

The outbreak began at the conclusion of the NAACP’s annual state convention. Attendees said several dozen people became sick on the morning of Oct. 26, a little more than 12 hours after a banquet dinner where roughly 300 people dined on salmon and salad. Twelve people were taken by ambulance to hospitals.

Hotel Sofitel spokeswoman Sandra Duhamel did not return a call Friday seeking comment. The hotel responded to the outbreak last week by shutting down its food service and conducting a property-wide cleaning. It had resumed partial food service by Monday.

The same hotel exposed guests to a similar outbreak in January 2008, when 62 people contracted norovirus after attending a Redwood City-San Mateo County Chamber of Commerce event. Guests dined that night on chicken and salmon.

County health investigators found several health code violations, including the use of unsanitary cloths to wipe down kitchen equipment, following the 2008 outbreak. The county did not find any violations on this occasion, Thaw said.

Norovirus is spread through contact with the vomit or feces of an infected person. Food service workers can spread the virus by not properly washing their hands before touching food.

People typically come down with norovirus symptoms between 12 and 48 hours after exposure, according to the CDC. The disease is most common in schools, health care facilities and other situations where people are grouped closely together. It causes 19-21 million illnesses per year in the United States, according to the CDC.

County and state health officials are continuing to investigate the cause of the outbreak, but the exact manner of transmission may remain a mystery.

“The problem with norovirus is it’s just so contagious,” Thaw said. “It could have been anybody.”